High-stability 1.5 μm external-cavity semiconductor lasers forphase-lock applications

Applications using phase-locked semiconductor lasers, such as homodyne detection, require lasers with narrow linewidth and high-frequency stability. The design and operating characteristics of two 1.5 μm external-cavity semiconductor lasers built for such applications are described. The measured beat linewidth is 4 kHz, and the spectral density of relative frequency noise deviates significantly from the intrinsic white spectrum only at frequencies below 4 kHz. It is estimated that this frequency jitter will induce approximately 1.1° RMS phase error in a second-order homodyne optical phase-lock loop that is optimized for the present beat linewidth     

Correlation between experiments to measure scattering centers in1.3 μm semiconductor diode lasers

Data obtained from two techniques on light scattering centers that are distributed along the length of the active region of 1.3-μm InGaAsP/InP diode lasers are presented and discussed. Light scattering characteristics were obtained for 16 lasers by analyzing light detected through the substrate (using spatially and polarization resolved electroluminescence) and by analyzing the facet emission for modulation features in the below-threshold reflectance-gain (R_m G_m) product. A Cartesian plot of the data shows the points to be dispersed about a best-fit line, but correlated. The failure to fall within experimental uncertainty on a line can be explained by a sampling phenomenon due to the discrete nature of the longitudinal modes and by the assumption of unequal or anisotropic scattering in the substrate and facet directions. The data are taken to show that the scattering is not isotropic     

Frequency-locked 1.3- and 1.5-μm semiconductor lasers forlightwave systems applications

A simple technique for frequency-locking 1.3- and 1.5-μm lasers to an excited-state atomic transition of noble gases using the optogalvanic effect is described. Many of the atomic transitions useful for these spectral regions are tabulated. The performance of frequency-locked lasers under direct frequency modulation is analyzed. It is shown that neither the frequency stability nor the receiver sensitivity shows any serious degradation when a frequency-locked laser is used in a frequency shift keying (FSK) transmission experiment     

Wavelength-dependent optimum output coupling enhances performanceof external-cavity-tuned semiconductor lasers at 1.5 μm

The output power of an external-cavity-tuned laser has been significantly increased by using an output coupler with a wavelength-dependent reflectivity. Our algorithm for determining the optimum wavelength dependence uses a simple gain model, and all necessary parameters can be determined from measurements of the same actual gain medium that will be used in the tunable laser source. This approach also increases tuning range     

Nonlinear gain and the spectral output of short-external-cavity 1.3μm InGaAsP semiconductor diode lasers

The effect of nonlinear gain on the steady-state spectral output of 1.3 μm InGaAsP semiconductor diode lasers was investigated by measuring the spectral output of lasers that were operated in a short external cavity (SXC). For the SXC lasers, an increase in the powers in both the long- and short-wavelength modes that are adjacent to the resonant mode (i.e., the mode that is resonantly enhanced by the SXC and hence lases strongly) was observed for output-power levels ⩾5 mW. These results suggest the presence of a symmetric-nonlinear-gain mechanism. Calculations that include a symmetric-nonlinear-gain mechanism correctly predict the observed trends in the evolution of the power in the longitudinal modes of an SXC laser with increasing output power. It is concluded therefore, that for strong single-mode oscillation and output powers above ≈5 mW such as found for an SXC laser operated well-above threshold, that the effects of a symmetric-nonlinear-gain mechanism are observable in the spectral output     

Low-threshold 1.5 μm compressive-strained multiple- andsingle-quantum-well lasers

Design considerations for low-threshold 1.5-μm lasers using compressive-strained quantum wells are discussed. Parameters include transparency current density, maximum modal gain, bandgap wavelength, and carrier confinement. The optical confinement for a thin quantum well in the separate-confinement heterostructure (SCH) and the step graded-index separate-confinement heterostructure (GRINSCH) are analyzed and compared. 1.5-μm compressive-strained multiple- and single-quantum-well lasers have been fabricated and characterized. As a result of the compressive strain, the threshold current density is loss limited instead of transparency limited. By the use of the step graded-index separate-confinement heterostructure to reduce the waveguide loss, a low threshold current density of 319 A/cm² was measured on compressive-strained single-quantum-well broad-area lasers with a 27 μ oxide stripe width     

Insights into carrier recombination processes in 1.3 μmGaInNAs-based semiconductor lasers attained using high pressure

The threshold current of 1.3 μm GaInNAs lasers increases by ~30%, up to a pressure of 1 GPa compared with a decrease of ~15% for Auger-dominated InGaAsP devices, indicating that direct band-to-band Anger recombination is not important in these materials. The lasing energy varies sub-linearly with pressure, indicative of the increasing interaction of the N-level with the conduction band     

Tensile-strained GaInAsP-InP quantum-well lasers emitting at 1.3μm

Tensile-strained GaInAsP-InP quantum-well (QW) lasers emitting at 1.3 μm are investigated. Low-pressure metalorganic chemical vapor deposition (LP-MOCVD) is used for crystal growth. High-resolution X-ray diffraction shows good agreement with theoretical simulation, photoluminescence spectra have good energy separation between light-hole and heavy-hole bands due to biaxial tension. The lowest threshold current density for infinite cavity length J_th/N_w ^∞ of 100 A/cm² is obtained for the device with -1.15% strain and N_w=3. The amount of strain which gives the lowest J_th/N_w^∞ experimentally clarified is around -1.2%. Threshold current of a buried-heterostructure (BH) laser is reduced to be as low as 1.0 mA. Enhanced differential gain of 7.1×10^-16 cm² is also confirmed by measurements of relative intensity noise. Much improved threshold characteristic with the feasibility of submilliamp threshold current can be achievable by optimizing the BH structure. The tensile-strained QW laser emitting at 1.3 μm with very low power consumption is attractive for the light source of fiber in the loop system and optical interconnection applications     

Circular beam emission from 1.3 μm InGaAsPstrained-multiquantum-well lasers

The beam divergence in the vertical direction from a graded index separate confinement heterostructure (GRINSCH) multiquantum-well (MQW) laser has been studied. It is demonstrated both theoretically and experimentally that a circular beam MQW laser can be produced by choosing appropriate thicknesses for the GRINSCH layers, while maintaining other desired laser characteristics. The beam divergence is found to be more affected by the index change induced by injected carriers than by strain in the MQW active layer. Theoretical results are in good agreement with the measurements for 1.3-μm InGaAsP strained MQW lasers     

High power semiconductor laser injection-locking at 1.3 μm

The injection-locking properties of a high power antireflection coated 1.3-μm slave laser subjected to relatively low injection powers from a distributed feedback (DFB) laser and from a tunable external cavity laser have been investigated. Narrow linewidth operation (~40 kHz) was demonstrated and the tuning range within two slave modes (~10 GHz) and over the gain profile (~40 nm) was investigated. In addition, the tracking properties of the slave laser for both frequency and phase modulated injected light was evaluated at 1 Gb/s, in which the fidelity was judged from bit-error-rate measurements. The maximum locked power under 1 Gb/s frequency modulation was about 145 mW, limited by the available master power; approximately 300 μW was injected into the slave     

Evidence for large monomolecular recombination contribution tothreshold current in 1.3 μm GaInNAs semiconductor lasers

The spontaneous emission, L, through a window in the substrate electrode of 1.3 μm GaInNAs MQW lasers was studied as a function of current, I, and temperature, T Close to room temperature, a characteristic temperature at threshold T,(L) T was observed as expected for band-to-band recombination in ideal quantum well devices. However, T ₀(I_th)≃T/3 indicating other processes occur. Analysis of the variation of L with I, reveals that monomolecular recombination contributes more than 50% to the total current at threshold and also that some Auger recombination may be present     

1.5 μm wavelength distributed reflector lasers with verticalgrating

A 1.5 μm wavelength distributed reflector laser, consisting of a distributed Bragg reflector rear facet and a distributed feedback region, was realised using deep-etching technology. A low threshold current of I_th=12.4 mA and a high differential quantum efficiency of η_d=42% from the front facet was achieved with a submode suppression ratio of 33 dB (I=2.4 I_th) for a fifth-order grating, 220 μm long and 6 μm wide device at room temperature     

Novel design of AlGaInAs-InP lasers operating at 1.3 μm

A novel design of AlGaInAs-InP lasers operating at 1.3 μm is proposed. A distinctive attribute of the proposed design is that the AlInGaAs active region is surrounded by an AlInAs electron stopper layer on the p-side and an InP hole stopper layer on the n-side. The stopper layers do not impede the carrier injection into the active region and at the same time reduce the thermionic emission of carriers out of the active region. Utilization of stopper layers allows one to increase the value of internal quantum efficiency and to select the waveguide material corresponding to the optimum optical confinement factor value     

High-performance λ=1.3 μm InGaAsP-InP strained-layerquantum well lasers

Compressively and tensile strained InGaAsP-InP MQW Fabry-Perot and distributed feedback lasers emitting at 1.3-μm wavelength are reported. For both signs of the strain, improved device performance over bulk InGaAsP and lattice-matched InGaAsP-InP MQW lasers was observed. Tensile strained MQW lasers show TM polarized emission, and with one facet high reflectivity (HR) coated the threshold currents are 6.4 and 12 mA at 20 and 60°C, respectively. At 100°C, over 20-mW output power is obtained from 250-μm-cavity length lasers, and HR-coated lasers show minimum thresholds as low as 6.8 mA. Compressively strained InGaAsP-InP MQW lasers show improved differential efficiencies, CW threshold currents as low as 1.3 and 2.5 mA for HR-coated single- and multiple quantum well active layers, respectively, and record CW output powers as high as 380 mW for HR-AR coated devices. For both signs of the strain, strain-compensation applied by oppositely strained barrier and separate confinement layers, results in higher intensity, narrower-linewidth photoluminescence emissions, and reduced threshold currents. Furthermore, the strain compensation is shown to be effective for improving the reliability of strained MQW structures with the quantum wells grown near the critical thickness. Linewidth enhancement factors as low as 2 at the lasing wavelength were measured for both types of strain. Distributed feedback lasers employing either compressively or tensile strained InGaAsP-InP MQW active layers both emit single-mode output powers of over 80 mW and show narrow linewidths of 500 kHz     

Dynamic behaviors of semiconductor lasers under strong sinusoidalcurrent modulation: modeling and experiments at 1.3 μm

The theoretical analysis is based on rate equations including gain-compression effects. General criteria are established to predict the existence of irregular behaviors. Experiments are performed on a single-mode buried-heterostructure InGaAsP laser at 1.3 μm. An original method is proposed to evaluate the parameters of the rate equations. Fully optical measurements are used. The nonlinear gain coefficient and the electrical response of the packaged laser are simultaneously determined from small-signal characteristics. Time-domain measurements show the three behaviors achieved with the laser, i.e., simple periodic, periodic with multiple spikes, and periodic doubling. Excellent agreement is found between experiments and calculations. Frequency-domain measurements are focused on distortions in periodic regimes. A quantitative limit of perturbation theories is given which corresponds to a second-order harmonic level exceeding -15 dB     

All-optical 1.5 μm to 1.3 μm wavelength conversion in awalk-off compensating nonlinear optical loop mirror

We demonstrate highly efficient all-optical conversion from 1.5 μm to 1.3 μm using a novel nonlinear optical loop mirror that compensates for walk-off. We make the fiber loop by splicing alternating segments of standard single-mode and dispersion-shifted fibers and choose their lengths such that the walk-off of the 1.3 μm and 1.5 μm pulses in one segment is completely reversed in the adjacent segment. We also show that the width of the converted pulses can be tailored by this scheme     

The carrier-induced index change in AlGaAs and 1.3 µm InGaAsP diode lasers

The carrier-induced index change has been measured for a large number of broad-area AlGaAs and 1.3 μm InGaAsP diode lasers. The observed index change with injected carrier density is-(1.2 pm 0.2) times 10^{-20}cm³for AlGaAs lasers and-(2.8 pm 0.6) times 10^{20}cm³for 1.3 μm lasers. The index change at threshold varies from -0.03 to -0.06 for AlGaAs lasers and from -0.04 to -0.10 for InGaAsP lasers. This variation is correlated with the carrier density at threshold, which depends on active layer thickness and doping level. For the first time, the observed index change is compared to the carrier density found from differential carrier lifetime measurements. This accurate determination of the carrier density represents a significant improvement over previous studies.     

High-speed performance of OMCVD grown InAlAs/InGaAs MSMphotodetectors at 1.5 μm and 1.3 μm wavelengths

A report is presented on the fabrication of high-speed In_0.53 Ga_0.47As metal-semiconductor-metal (MSM) photodetectors incorporating a high-quality lattice-matched InAlAs barrier enhancement layer, grown by organometallic chemical vapor deposition (OMCVD). Fast responses of ~55 ps full-width half-maximum at 1.5 μm and ~48 ps at 1.3 μm wavelengths are observed, corresponding to intrinsic device bandwidths of ~8 GHz and ~11 GHz, respectively. The absence of any tail to the pulse response, and of any low-bias DC gain, indicates a low-trap density at the InAlAs/InGaAs heterointerface. Bias independent dark currents of 10-20 μA are observed below breakdown, which occurred at >30 V in devices with a 500-A-thick InAlAs layer     

Tuning ranges for 1.5 μm wavelength tunable DBR lasers

Tuning ranges for 1.5 μm wavelength tunable distributed Bragg reflector (DBR) lasers were studied experimentally. Over 10.0 nm (1.25 THz) quasicontinuous tuning under 5 mW light output conditions and 4.4 nm (550 GHz) continuous tuning at 1 mW were achieved     

Low-threshold-current-density 1.5 μm lasers using compressivelystrained InGaAsP quantum wells

A low-threshold current density (J_th) of 140 A/cm² for broad-area 1.5-μm semiconductor lasers with uncoated facets is demonstrated at a cavity length of 3.5 mm. This was achieved by the use of a single InGaAsP quantum well (QW) of 1.8% compressive strain inside a step-graded InGaAsP waveguide region. Low-cavity losses of 3.5 cm^-1 and a relatively wide quantum well as compared to InGaAs wells of equivalent strain contribute to this high performance. Double QW devices of 2 mm length showed threshold current densities of 241 A/cm². Quaternary single and double QWs of similar width but only 0. 9% strain gave slightly higher threshold current density values, but allowed growth of a 4 QW structure with a J_th of 324 A/cm² at L=1.5 mm